That's awesome. It was always some weird situation with abstract JVM specification but only one de-facto standard implementation which everyone used. I hope J9 will be a viable alternative, so everyone wins.
I've been a Java dev for a decade, and I never once used anything but Sun / Oracle's version (now OpenJDK, which is almost the same thing).
I really have no idea who ever used these other JVM implementations. I'm guessing it would be companies with specialized hardware such as IBM mainframes or finance firms paying a lot of money to squeeze out performance with WebLogic. Either because of poor marketing or some other reason, no developer or company I worked with had a reason to use anything else.
Back in the early 2000s, the IBM JVM was freely available and frequently used as a drop-in replacement for the Sun JVM, since it was often faster and less memory-hungry.
IBM also had an open-source Java compiler written in C++, Jikes [1], that was considerably faster than Sun's javac. However, it was eventually abandoned. Jikes is coincidentally also the name of IBM's open-source research JVM [2], which is still under development.
Azuul is apparently popular in areas requiring low latency, such as financial trading.
As an aside, there are many niches that most people haven't heard about. You might be surprised about all the kinds of software that are hiding under rocks — invisible to most people because they're not working in that industry. Things like MUMPS, K/kdb, Fortran, Delphi — lots of obscure stuff that has left the mainstream (or never entered it in the first place) but is still in use.
Jikes RVM was known as Jalapeño first, then they changed the name to Jikes RVM due to a name clash (Jikes Java compiler already existing at the time). I'm not sure it is still active though, a lot of researchers left IBM a few years ago (2012) and Java ceased to be the hot topic anyways.
I used JRockIT for some stuff. We found it to beat HotSpot (at the time) in some of our use cases. Eventually they got brought in house and HotSpot gained those speed improvements. One of the things that was really useful with it was Mission Control, which didn't exist to the same degree with the HotSpot VM.
HP had PA-RISC and Itanium so they ported Sun/Oracle's JVM (presumably under some commercial license agreement as this was before OpenJDK) to those platforms and distributed/supported it with their HP-UX. Oracle DBs on Itanium used that port of the JVM too IIRC.
